Burnable neutron absorbers

ABSTRACT

A neutron-absorber body for use in burnable poison rods in a nuclear reactor. The body is composed of a matrix of Al 2  O 3  containing B 4  C, the neutron absorber. Areas of high density polycrystalline Al 2  O 3  particles are predominantly encircled by pores in some of which there are B 4  C particles. This body is produced by initially spray drying a slurry of Al 2  O 3  powder to which a binder has been added. The powder of agglomerated spheres of the Al 2  O 3  with the binder are dry mixed with B 4  C powder. The mixed powder is formed into a green body by isostatic pressure and the green body is sintered. The sintered body is processed to form the neutron-absorber body. In this case the B 4  C particles are separate from the spheres resulting from the spray drying instead of being embedded in the sphere.

REFERENCE TO RELATED APPLICATIONS

This application relates to an application Ser. No. 352,686, filedconcurrently herewith to Kenneth C. Radford and W. George Carlson forBurnable Neutron Absorbers, assigned to Westinghouse ElectricCorporation (herein referred to as Radford Application). Radfordapplication and applications Ser. No. 334,720 filed Dec. 28, 1981 toKenneth C. Radford for "Neutron Absorber Pellets with ModifiedMicrostructure" and Ser. No. 915,691 filed June 15, 1978 to W. L. Orr etal. for "Low Reactivity, Penalty Burnable Poison Rods" are incorporatedbelow by reference.

BACKGROUND OF THE INVENTION

This invention relates to the art of nuclear reactors and it hasparticular relationship to burnable neutron-absorber assemblies, alsocalled burnable-poison assemblies, for nuclear reactors. The burnableneutron-absorber assemblies with which this invention concerns itselfare of the type described in Radford application. Such neutron-absorberassemblies include annular ceramic pellets which are stacked in tubesinserted in the core of a reactor. It is with the pellets that thisinvention concerns itself. Such a pellet includes a matrix of arefractory material which may include aluminum oxide (Al₂ O₃) orzirconium oxide ZrO₂ or a combination of the two. A neutron absorber orneutron-capture component is distributed throughout this matrix. Theneutron absorber may include one or more elements or compounds of themetals boron, gadolinium, samarium, cadmium, europium, hafnium,dysprosium and indium. A neutron absorber commonly used is boron carbide(B₄ C) either natural or with the boron enriched B¹⁰.

In the interest of brevity and concreteness to facilitate theunderstanding of those skilled in the art in the practice of thisinvention, this application will deal specifically with a matrix of Al₂O₃ and a neutron absorber of B₄ C. It is understood that to the extentthat this invention is practiced with other materials, such practice iswithin the scope of equivalents of this invention as scope ofequivalents is defined and described in the Supreme Court Grover casecited in Radford application.

The method of producing pellets disclosed in Radford application and thepellets produced thereby have proven themselves highly satisfactory.However, experience with this method and the pellets produced therebyhad led to the conclusion that several improvements are desirable. It isdesirable that the pores or voids in the matrix be more efficiently oreffectively used to take up the expansion of the B₄ C and absorb thehelium gas generated by the neutron-boron reaction. It is also desirablethat the strength, particularly the compressive strength, of the matrixbe improved. It is an object of this invention to provide a method forproducing neutron-absorbing bodies or ceramics having theabove-described desirable properties. It is also an object of thisinvention to provide a neutron-absorbing body or ceramic having theabove desirable properties.

In this application the expression "neutron-absorber assembly" or"poison assembly" means the neutron-absorber structure or rod as a wholeincluding the pellets and the container in which the pellets arestacked; "neutron absorber" means the neutron-capture component, e.g.,B₄ C; "neutron-absorber body" means the body including the neutronabsorber in its matrix.

SUMMARY OF THE INVENTION

In the practice of the invention of the Radford application a slurry ofa mixture of Al₂ O₃ and B₄ C powder are spray dried. The resulting driedpowder consists of agglomerated spheres of Al₂ O₃ in which B₄ Cparticles are embedded. This powder is then pressed into pellets andsintered. It has been realized in arriving at this invention that theneutron-absorbing effectiveness and the resistance to swelling of theceramic or neutron-absorber bodies can be improved and at the same timethe strength of the ceramic bodies can be increased by separating theAl₂ O₃ and the B₄ C in the production of the ceramic bodies.

In the practice of this invention a slurry of the Al₂ O₃ alone isproduced. A hard binder, typically polyvinyl alcohol, is added and theslurry and binder are spray dried. The product of the spray drying is apowder of agglomerated Al₂ O₃ spheres 30 to 50 microns in mean diameter.This powder is mixed with dry B₄ C powder 5 to 15 microns in mean sizeforming a homogeneous mixture. This mixture is pressed isostaticallyinto green tubes which are then sintered. When the mixture is pressedthe agglomerates of Al₂ O₃ deform and lock together trapping the B₄ Cparticles in the pores. During sintering, the binder volatilizes and thestructure of the resulting ceramic has nearly spherical high-densityregions of Al₂ O₃. These regions are predominantly surrounded by poresand by B₄ C particles.

The practice of this invention results in a preferred location in theAl₂ O₃ matrix of the B₄ C particles and the pores. The matrix of the Al₂O₃ consists microscopically of high-density polycrystalline regions andits strength is higher than for the matrix produced in the practice ofthe invention of Radford application. Since the Al₂ O₃ is dried, thehygroscopic tendency of the matrix is materially reduced. The B₄ Cparticles are predominantly in the pores of the matrix. The availableporosity accommodates the swelling of the B₄ C particles when bombardedby neutrons and the resulting helium gas.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of this invention, both as to itsorganization and as to its method of operation, together with additionalobjects and advantages thereof, reference is made to the followingdescription taken in connection with the accompanying drawings, inwhich:

FIG. 1 is a flow chart illustrating the practice of this invention;

FIG. 2 is a photomicrograph of a ceramic or neutron-absorber bodyproduced in the practice of this invention; and

FIG. 3 is a photomicrograph of a ceramic body produced in the practiceof the invention of Radford application presented for comparisonpurposes.

DETAILED DESCRIPTION OF THE INVENTION

In the first step 11 of the process of this invention a powder of Al₂ O₃is milled in a ball mill in a liquid, typically water which may bedeionized. Small but effective quantities of a wetting agent, asurfactant and a deflocculant are added to the water and Al₂ O₃. Themean size of the Al₂ O₃ is 10 to 30 microns. The relative quantities ofthe Al₂ O₃, the water and the other components are substantially thesame as disclosed in the Radford application. The result of the millingis a slurry containing about 40% Al₂ O₃ only.

In the second step 13 a hard binder, such as polyvinyl alcohol, is addedto the slurry. In the third step 15 the slurry is spray dried inapparatus as disclosed in Radford application. The spray drying resultsin spheres of agglomerated particles of Al₂ O₃ having a mean diameter ofabout 30 to 50 microns. In the fourth step 17 this powder is screened toeliminate excessively large agglomerates. In the next step 19 ahomogeneous mixture of the Al₂ O₃ agglomerates and B₄ C powder isproduced. The content of the B₄ C powder in this mixture in weightpercent may be between 1 and 50. The mean size of the B₄ C particles isbetween 5 and 15 microns.

The remaining steps 21 to 31 are the same as the corresponding steps ofRadford application. The homogeneous mixture is poured into a mold, step21. A green cylinder or green mass is formed by compressing the powderin the mold by isostatic pressure, step 23. Optionally the greencylinder may be presintered, step 25. The mass is sintered to size, step27. The sintering is in an atmosphere of argon at about atmosphericpressure and the sintering temperature is between 1400° C. and 1800° C.The outer surface of the sintered body is ground, step 29. Ceramicneutron-absorber pellets of B₄ C in a matrix of Al₂ O₃ are cut from thecylinder.

The microstructure of a ceramic body produced in the practice of thisinvention is shown in FIG. 2. As indicated a length of about 1/16 inchon the photomicrograph corresponds to 5 microns. The black areas 33 onthe photomicrograph are reproductions of the pores, the dark-gray areas35 of the B₄ C. The regions of Al₂ O₃ alone are interlocked as appearsat 39. The B₄ C regions are in pores surrounding the Al₂ O₃ as appearsat 41.

The photomicrograph shown in FIG. 3 is illustrative of the practiceprior to this invention and is presented for comparison purposes. Thisphotomicrograph also shows black areas 33 corresponding to pores,dark-gray areas 35 corresponding to Al₂ O ₃ and light-gray areas 37corresponding to B₄ C. But the interlocked regions of Al₂ O₃ alone areabsent. Nor is the B₄ C in pores encircling the Al₂ O₃. The B₄ C asshown in FIG. 3 intermingles with the Al₂ O₃.

While preferred practice and a preferred embodiment of this inventionare disclosed herein, many modifications thereof are feasible. Thisinvention is not to be restricted except insofar as is necessitated bythe spirit of the prior art.

We claim:
 1. The method of making burnable neutron-absorber bodies forthe burnable-poison assemblies of a nuclear reactor which comprises:(a)producing a slurry of a powder of a refractory material including one ormore of the class consisting of Al₂ O₃ and ZrO₂ ; (b) adding a binder tosaid slurry; (c) drying said slurry to produce a powder of agglomeratedparticles of one or more of the class of Al₂ O₃ and ZrO₂ including saidbinder; (d) mixing said powder with a powder of the class ofneutron-absorbers consisting of elements or compounds of boron,gadolinium, samarium, cadmium, europium, hafnium, dysprosium and indium,to form a mixture of said powders; (e) isostatically compressing saidmixture to form a green body; (f) sintering said green body to form asintered body; and (g) forming said sintered body into aneutron-absorber body of appropriate shape and dimensions.
 2. The methodof claim 1 wherein the mean size of the aluminum oxide powder in theslurry is about 10 to 20 microns and the mean size of the powder of theneutron absorber is about 5 to 15 microns.
 3. The method of claim 1wherein the slurry is spray dried producing aluminum oxide spheres of 30to 50 microns mean diameter.
 4. The method of claim 1 wherein theneutron absorber in the mixture of powders is boron carbide (B₄ C) andthe content of the B₄ C in the mixture in weight percent is about 1 to50.
 5. The method of claim 1 wherein the green body is sintered in anatmosphere of argon at about atmospheric pressure at a temperature ofbetween 1400° C. and 1800° C.
 6. The method of claim 1 wherein thebinder is a hard binder.
 7. A burnable neutron-absorber body for use inthe burnable-poison assemblies of a nuclear reactor, said body beingformed of a porous matrix of Al₂ O₃, said matrix including high-densitypolycrystalline particles of Al₂ O₃ juxtaposed to pores in which poresthere are particles of B₄ C.
 8. The body of claim 7 wherein the poressubstantially encircle the Al₂ O₃ particles, said encircling porespartially or wholly containing particles of B₄ C.